Unshaped refractories and non-burning refractories generally obtain construction strength by adding a binder. Consider the change of the binder during the heating process and the reaction between the binder and the bonded material at high temperature, and the influence on the composition, microstructure and performance of the bonded material.
There are many kinds of binders for monolithic refractory materials, and there are also many classification methods, such as classification according to chemical composition and properties, classification according to hardening conditions, and classification according to bonding mechanism. The most widely used binder is the aluminate cement binder.
01
The binding mechanism of calcium aluminate cement
Calcium aluminate cement is currently the most widely used binder for monolithic refractories. There are many ways to synthesize calcium aluminate cement. Generally, alumina (or high-quality bauxite) and calcium carbonate are used as raw materials through calcination or electrofusion, and calcium carbonate is replaced by dolomite to obtain magnesium-aluminum spinite. Calcium aluminate cement of stone, and high iron calcium aluminate cement made by melting high iron bauxite and limestone as raw materials. Although CaO in calcium aluminate cement has an adverse effect on the high temperature performance of refractory materials, it can make the material obtain higher strength in a short period of time (6~24h), and is beneficial to the performance of castables. Stable and uniform, so it is still widely used.
The hydration mechanism of calcium aluminate cement is that after the CA, CA2, and C12A7 in the cement are mixed with water, the Ca2+ and Al(OH)4- ions rapidly dissolve into the water to form an aqueous solution. The aqueous solution quickly reaches saturation, and after reaching saturation, crystal nuclei are precipitated from the aqueous solution to form hydration products. These hydration products will be interconnected to form a staggered network structure to solidify and harden, so that the material gains strength. In fact, this is a process of dissolution→saturation→precipitation of the hydration phase.
02
Binding mechanism of aluminum dihydrogen phosphate
Phosphate-based binder is a traditional binder with excellent performance. It was first used by relevant departments of the aerospace industry to manufacture heat-resistant coatings and as an adhesive for component packaging in the electronics industry. In the production and construction of refractory materials, phosphate is also widely used as a binder for unshaped refractory materials and non-burning refractory products. Among the phosphate binders, aluminum phosphate is the most widely used. Aluminum phosphate binders are usually prepared by reacting phosphoric acid with more active aluminum hydroxide. Different, the product of the reaction is also different, when the molar ratio of P2O5 and Al2O3 is 3, the product obtained by the reaction is aluminum dihydrogen phosphate. The viscosity of phosphate aqueous solution has a great relationship with the M value of phosphate, the larger the M value, the greater the viscosity of its aqueous solution.
The chemical formula of aluminum dihydrogen phosphate is Al(H2PO4)3, which is generally a white powdery solid or a colorless and odorless viscous liquid, which is easily soluble in water. The aqueous solution of aluminum dihydrogen phosphate has a certain viscosity and can The effect is that the material has a certain strength. Aluminum dihydrogen phosphate will undergo a series of chemical reactions after heating.
The strength obtained by the material with aluminum dihydrogen phosphate as the binder is different when the material is treated at different temperatures. When the reaction temperature is lower than 800 ° C, the main polymerization reaction of aluminum dihydrogen phosphate occurs, and the product of the reaction is aluminum metaphosphate Polymer [Al(PO3)3]n, when the reaction temperature is 800~1200℃, aluminum metaphosphate polymer decomposes into aluminum phosphate (AlPO4) and phosphorus pentoxide (P2O5), due to the large amount of AlPO4 generated, the material Strength is improved. When the temperature is higher than 1400°C, Al2O3 and gaseous P2O5 will be decomposed, and the porosity of the material will increase, but because the formation of activity can promote the high temperature sintering of the material, the compressive strength of the material will be greatly improved. If the temperature continues to rise, the amount of liquid phase formed between the basic compound in the raw material and Al2O3-SiO2 will increase, resulting in a decrease in the compressive strength of the material.